The success of joint replacement surgery is primarily dependent on two factors, namely, (1) the position of the implant components with respect to the boney anatomy; and (2) the postoperative state of the surrounding soft tissues. Joints are surrounded by ligamenteous and capsular tissue. The state of these soft tissues determines the laxity and stability of the joint. The state of the soft tissues is effected by the following two factors: (1) the position of the components; and (2) the amount of soft tissue releases performed by the surgeon, as discusses in U.S. patent application publication No. 2005/0267485, which is hereby incorporated by reference in its entirety. In addition, a number of devices and techniques have been described that attempt to facilitate ligament balancing during a TKA procedure and in particular, some of these techniques involve trial prosthesis components which are used after femoral and tibial bone cuts are made to assess ligament tissue. See, U.S. Pat. No. 5,733,292, which is hereby incorporated by reference in its entirety.
Other devices are used to measure a gap between the distal femur and proximal tibia in extension and to help a surgeon recreate that same gap when the knee is in flexion. See, U.S. patent application publication No. 2003/0187452 and U.S. Pat. No. 6,575,980, both of which describe “gap checking” devices, each of which is hereby incorporated by reference in its entirety. Other devices have been developed to help measure an amount of ligament tension or to apply a desired amount of tension to the ligaments and in addition, paddle-like devices have been suggested for applying or measuring tension across a knee joint. See, U.S. Pat. Nos. 4,501,266; 5,597,379; 5,540,696; 5,800,438; 5,860,980; 5,911,723; and 6,022,377, each of which is hereby incorporated by reference in its entirety.
The device described in the above mentioned '485 publication is designed to determine the rotational alignment of the femoral component such that the knee is in optional tension; however, there are a number of disadvantages and limitations associated with this device. For example, the following are disadvantages associated with this device: (1) the distal femoral cut must be made first before the device is inserted and therefore, one can not change the planning in varus/valgus; proximal/distal, and flexion/extension; (2) the device must be fixed to the femur and therefore, requires bone screws on the medial and lateral sides which add invasiveness to the bone, as well as the soft tissues since access is required to the lateral side of the joint; (3) a vast number of components sizes for the tibia and the femur are still required; and (4) the system has constraints due to it not being able to account for different prosthetic designs using the same components, for example, different degrees of constraint or concavities of the tibial or femoral components.
In conventional surgery, trial reductions are often performed as a final check as to whether or not the final placement of each component of the implant is satisfactory for each specific patient. This step is performed after planning the position of the implants and the bone cuts on the tibia and femur are made. The surgeon temporarily implants a set of so called trial components, which are the same shape and size of the final prosthesis to be implanted, onto the bone cut surfaces and reduces the joint. With the trial components in place, the surgeon can verify that the stability, kinematics, and orientation of the joint are satisfactory by performing a series of tests. If a satisfactory result is not obtained for any of the tests, the surgeon has the option of either performing releases of the soft tissues, or to make one or several re-cuts on the bone in an attempt to adjust the position or the size of the prosthesis. However, this is often not an easy or obvious task because irreversible bone cuts are already made and therefore, the position of the components can only be altered in a limited manner. For example, it is very difficult to increase the size of a distal femoral component or to repair an anterior cut that notches the femoral bone. Correcting the position of the implants after the cuts have already been made can therefore be a time consuming and frustrating process. Soft tissue releases can be made to help compensate for poor component positioning in some cases; however, this is clearly not the optimal solution. Another disadvantage of having to perform trial reductions is that the hospital is required keep several trays of trial components available in the operating room to accommodate all the different sizes and shapes of implants. This increases the cost of the associated surgical instrumentation as well as the cleaning, sterilization and storage costs. The additional clutter around the operating table can also pose logistic issues for the staff.
While there are some systems and methods for determining the position of a knee-joint endoprosthesis, each suffers from a number of deficiencies. For example, U.S. patent publication No. 2005/0251148 to Friedrich et al. discloses a system that determines the femur and tibia by calculating various virtual relative positions of the femur and tibia according to geometrical data of the knee-joint endoprosthesis and to different assumed positions of the tibial part on the tibia and/or of the femoral part on the femur when the knee is straightened and bent, and in which an assumed position in which the virtual relative position of the femur and the tibia when the knee is straightened and bent differs from the spread position in a specified manner is determined as a selected position. However, this system fails to include a control system that includes a device that moves one bone and is linked to a controller to cause controlled movement of the bones during movement of the joint and in view of the measured/calculated information to monitor the virtual relative positions of the femur and tibia.
A system that would therefore allow the surgeon to perform a “virtual” trial reduction before the actual bone cuts are made and without all the required trial components would be of great value.